Update PIXI.d.ts generateTexture

Should fix phaserjs#1719 ?

Author: clark-stevenson

typescript/pixi.d.ts

@@ -492,7 +492,7 @@ declare module PIXI {
         displayObjectUpdateTransform(): void;
         getBounds(matrix?: Matrix): Rectangle;
         getLocalBounds(): Rectangle;
-        generateTexture(resolution: number, scaleMode: scaleModes, renderer: PixiRenderer): RenderTexture;
+        generateTexture(resolution?: number, scaleMode?: number, renderer?: PixiRenderer): Texture;
         mousedown(e: InteractionData): void;
         mouseout(e: InteractionData): void;
         mouseover(e: InteractionData): void;
@@ -639,6 +639,7 @@ declare module PIXI {
         drawShape(shape: Ellipse): GraphicsData;
         drawShape(shape: Polygon): GraphicsData;
         endFill(): Graphics;
+        generateTexture(resolution?: number, scaleMode?: number): Texture;
         lineStyle(lineWidth?: number, color?: number, alpha?: number): Graphics;
         lineTo(x: number, y: number): Graphics;
         moveTo(x: number, y: number): Graphics;